The present invention relates to optical waveguides. More particularly, the present invention is directed toward forming optical waveguides from optical fibers, which are suitable for use in data communication.
To minimize insertion loss, the loss of optical energy when coupling data links in fiber-optic communication systems, it is important to correctly match the aperture through which optical energy is transmitted with the aperture through which optical energy is detected. As a result, the areas of the apertures must be correctly sized and aligned.
The ideal interconnection of one fiber to another would have two fibers that are optically and physically identical and held by a connector that aligns the fibers so that the interconnection does not exhibit any influence on light propagation therethrough. Formation of the ideal interconnect is difficult for several reasons. These include variations in fiber properties, tolerances in the connector, as well as in cost and ease of use.
Commercially available interconnection devices have typical insertion losses from between 0.2 dB to 4 dB. This range of insertion loss results from several factors that may be divided into those related to fibers and those related to interconnection devices. Fibers intrinsically contribute loss to an interconnection and any fiber has variations that are produced during manufacture. These variations exist not only among different lots of fibers, but also within a length of a single fiber, as well. The main variations in these cases are in the core and cladding diameters and fiber numerical aperture (NA). The core ellipticity, cladding ellipticity, and core-to-cladding eccentricity exacerbate the problems associated with variations in the core and cladding diameters. Losses caused by diameter variations, NA variations, eccentricity, and ellipticity are intrinsic to the fiber and the total loss contributed by these intrinsic factors can range from less than 0.2 dB to over 2 dB, depending on how well two fibers match.
Connector-related losses may also arise even when there are no intrinsic variations in the fibers. These types of losses arise when two fibers are not aligned on their center axes and lateral or axial displacement can be, and usually is, the greatest cause of loss in the connection. For example, a 0.5 dB loss that is due to a displacement, equal to 10% of the core diameter, will require tolerances to be maintained on each connector (fiber) that is within 2.5 xcexcm. Tolerances of this magnitude are difficult to achieve. Add to this, the losses that are also induced due to angular misalignment, and the total tolerances that must be maintained in the termination process, proper fiber and/or connector end preparation becomes problematic.
The considerations discussed above with respect to fiber-to-fiber interconnections apply equally to fiber-source and fiber-detector interconnections, as well. The result is that the requirements that should be achieved to provide efficient optical coupling necessitate end-finishing or termination processes that strives to provide lossless propagation of optical energy. To that end, it is desired to provide the end of a fiber that functions as either a transmission or reception aperture with a smooth end finish free of such defects that may change the geometrical propagation patterns of optical energy passing therethrough. These defects include hackles, burrs, fractures, bubbles and other contaminants.
Preparation of conventional glass optical fibers employs score-and-break techniques or mechanical polishing techniques. The score-and-break technique provides an optical fiber with an arc that is scored. Tension is applied to that optical fiber so that the score propagates across the width of the optical fiber, segmenting the same. This technique is capable of producing an excellent cleaved end. Repeatability, however, it is difficult, lowering yields and increasing the cost of the finished optical fibers. In addition, a great amount of skill is required to properly control both the depth of the scoring and the amount of tension during breaking. The aforementioned control may be frustrated by intrinsic fiber variations. Finally, the difficulty in controlling both the depth of scoring and breaking tension increases as the length of the optical fiber becomes shorter.
Polishing, compared to scoring-and-breaking, has the advantage of consistent results, but is a much more costly technique. Polishing is typically performed after a connector, or ferrule, has been attached to the optical fiber. Polishing a bare optical fiber is impractical. Usually, a polishing fixture is provided that controls the polishing to a fixed dimension, e.g., usually within 0.3 xcexcm.
Polymer-based optical fibers may be segmented with a sharp, and preferably hot, blade. As with the polishing technique mentioned above with respect to glass optical fibers, segmenting is performed on polymer-based optical fibers after a connector has been attached. Polymer-based optical fibers may also be polished, but it is very difficult to achieve the performance of a glass or quartz optical fiber.
In addition to providing a smooth end finish, the preparation procedure should provide the optical fiber with a cleaved end, i.e., the end of the optical fiber is typically planar and lies in a plane with the longitudinal axis of the optical fiber extending orthogonally thereto. Otherwise, an angle may exist between the axes of juxtaposed fibers and fiber-devices, referred to as tilting. Tilting can cause additional, and sometimes quite severe, losses in addition to those mentioned previously. While tilting loss can be controlled to some degree by proper end preparation and positioning of adjacent fiber ends, it should not be completely ignored. Often alignment mechanisms are employed to reduce the effects of tilting. Such alignment mechanisms include lenses that may be effectively coupled and aligned, (with minimum loss to the end of the optical fiber).
Referring to FIG. 1, a fiber-to-fiber arrangement 10 employing lensed optical fibers 12 and 14 is shown. The lenses are shown as 12a and 14a, at the ends of optical fibers 12 and 14, respectively. Lenses 12a and 14a are typically spherical and refract optical energy, shown as 12b and 14b, propagating therethrough to facilitate control of the path of light therebetween. In this manner, the lateral and axial alignment between optical fibers 12 and 14 may be relaxed. However, optical fibers 12 and 14 should be accurately placed and aligned behind the lenses in order to actually see any real or significant benefits to the overall loss considerations (e.g., low losses). Moreover, such conditions are most often achieved without the aid of non-integral support elements such as lenses, when the appropriately prepared fiber ends are perpendicular to the fiber axis. One manner in which to form lenses 12a and 14a is discussed below with respect to a fiber-source arrangement.
Referring to FIG. 2, shown is a fiber-source arrangement 16 in which a lens 20 is formed on one end of an optical fiber 18. The source-fiber arrangement 16 includes an optical fiber 18 composed of a core 18a and a cladding 18b. A lens 20 is formed at an end of a fiber core. Were optical fiber 18 formed from silica glass, lens 20 would be formed in the following manner: First, while a portion of the silica glass optical fiber 18 is heated by heating means such as a burner, a tensile force is applied to the fiber in the longitudinal direction thereof, whereby the heated portion extends. When the outer diameter of the heated portion has decreased to a predetermined diameter, optical fiber 18 is cut at the diameter-reduced portion, and then the cut end is again heated for fusion. In the heating step, extreme end 22 of optical fiber 18, including core 18a in the center thereof, becomes spherical in shape due to surface tension, and this spherical end functions as a lens. Thus, lensed optical fiber 18 has a taper portion 24 extending from extreme end 22 to an outer peripheral edge, which is not affected by heat, and having a certain inclination determined by the heating and drawing conditions. Lensed optical fiber 18 produced in this manner is optically connected to a semiconductor laser 26, and a laser beam 28 is emitted from a light-emitting surface 30 of semiconductor laser 26. In this case, laser beam 28 radiates in conical form. Laser beam 28 is incident on extreme end 22 at the extremity of core 18a and is propagated through core 18a, as indicated by the arrows in FIG. 2, and is used for optical communications. A drawback with the prior art attempt of lens formation is that artifacts are produced by the thermal energy propagating through the optical fiber 18. These artifacts may lead to increased insertion loss.
What is needed, therefore, is a technique to thermally shape an optical fiber while reducing formation of artifacts.
Provided are a thermally-shaped optical fiber and a method for forming the same that features creating a flow of thermal energy between two spaced-apart regions of the optical fiber. The flux of thermal energy in the flow is substantially constant to define a graded index of refraction in a portion of the optical fiber located between said two-spaced apart regions. This minimizes formation of unwanted optical artifacts in the portion. For example, a graded index of refraction is formed in the portion, thereby avoiding abrupt changes in the variation of the index of refraction in the portion. Additionally, the formation of a self-focusing lens in the portion is minimized, if not abrogated. Both of the aforementioned optical artifacts, abrupt changes in indices of refraction and the self-focusing lens, leads to insertion loss. By avoiding formation of these optical artifacts, the insertion loss of the optical fiber is greatly reduced, if not completely absent.